chore: bump CALIBRE_VERSION from 9.14.0 to 9.15.0

[github-actions]

The Dockerfile is pinned to CALIBRE_VERSION=9.14.0, but kovidgoyal/calibre has a newer release available.

Current version: 9.14.0
Latest version: 9.15.0
Upstream release: https://github.com/kovidgoyal/calibre/releases/tag/v9.15.0
Dockerfile source: https://github.com/kovidgoyal/calibre/releases

Update the Dockerfile version arg if the new upstream release is compatible.

---

dependency: kovidgoyal/calibre
arg: CALIBRE_VERSION
from: 9.14.0
to: 9.15.0